In modern culture, it is not considered socially acceptable for married people to have extramarital sexual partners, but these Amazonian cultures beg to differ. 

These tribes hold the belief that when a woman gets pregnant, all her sexual partners are considered part-biological fathers, and having multiple partners contributes to the child inheriting the best genes.

This simply means a woman must engage in sexual relations with multiple men, as they think that the seed from each man forms in the mother's womb to create a single child. This is also referred to as partial or multiple paternity.

A new study published in the Proceedings of the National Academy of Sciences has found that up to 70% of Amazonian cultures may have believed in the principle of multiple paternity.

Partible paternity in Amazon

In many Amazonian tribes, all men who have been with a woman during her pregnancy are considered fathers and share the responsibility of raising the child.

In some other tribes, all children have multiple fathers, while in others, a child has one primary father, but partible paternity is still accepted. Even more surprising is that jealousy doesn't seem to be a problem. Husbands in these societies have been raised by multiple fathers, reinforcing the belief that having multiple fathers benefits a child. So, while other communities may find this practice weird, it's a norm in these parts.

Partible paternity wasn't just about reproduction; it strengthened family bonds, as brothers often shared wives in some of these cultures. Children were raised by a community of fathers, and their upbringing benefits from the contributions of multiple men. 

"In some of these Amazonian cultures, according to Robert Walker, assistant professor of Anthropology in the College of Arts and Science, it was considered strange if you did not have multiple sexual partners. Cousins were often preferred partners, so it was especially rude to shun their advances."

Walker explains that sexual promiscuity was a normal and accepted practice in many of these societies, and married women explored their sexual freedom unashamedly.

This practice has been a longstanding tradition within Amazonian tribes and continues to be practised to this day.